The present invention relates to date display devices intended to be fitted to timepieces of the type including a movement and hands driven by the movement and displaying the time.
In the first watches of this type, the date was displayed by an additional hand making one revolution in 31 days. One of them, dating from the 18th century, is described at page 156 of the catalogue of selected works of the International Horological Museum in La Chaux-de-Fonds (ISBN 2-940088-07-1). It was thus an analogue type display. Such a solution is simple, but can only be read approximately, since the angle traveled by the hand each day is insufficient to allow two neighboring positions to be differentiated at a glance.
In most of the watches currently marketed, for example those shown at page 281 of the aforementioned catalogue, the display occurs in a digital manner, by means of a disc bearing the numbers from 1 to 31 and jumping forward each day. The date appears in a window made in the dial. The figures displaying this information have to be of small dimensions, and are thus difficult to read.
One solution to this problem consists in providing the device with first and second display members mounted so as to move in rotation on the movement, arranged under a dial provided with apertures. These members are respectively intended to display the units and the tens of the date by means of figures, visible through the apertures. The device further includes a drive and positioning mechanism controlled by the movement and arranged so as to cause one or the other or both of the members to jump forward once a day.
Such a device is, for example, disclosed in Swiss Patent No. 310 559. This device is controlled by a clockwork movement, by means of a wheel making a revolution in 24 hours. The latter drives through 1/31st of a revolution, and once a day, a wheel assembly including two superposed wheels. The display members are formed of two discs arranged side by side, one displaying the tens of the date and the other the units, the current date appearing in an aperture of the dial.
The wheel assembly and the discs are arranged so that, when the units figure of the date displayed is equal to 0 or comprised between 2 and 8, only the units disc is driven, when the units figure is equal to 9, the units and the tens discs are both driven, when the units figure is equal to 1 and the tens figure to 0, 1 or 2, only the units disc is driven, and when the units figure is equal to 1 and the tens figure to 3, only the tens disc is driven. Such a device allows the date to be displayed by means of figures of large dimensions.
It is evident that the date is easier to read, but less aesthetically pleasing, the more apparent it is, since the figures overload the dial.
Another solution is disclosed in European Patent No. 0 619 035. It relates to an electronic watch of the analogue type, wherein the date is displayed by means of two hands. The first indicates the tens, in four positions making an angle of 90xc2x0 between them, while the second displays the units, in ten positions regularly distributed over one revolution.
This solution can only be envisaged insofar as figures are located facing the position of the hands. This is necessary to permit reading, since the hands do not occupy a usual position for a given value, as is the case of the hour and minute display. This solution thus loads the dial considerably, without however making it simple to read.
The object of the present invention is to allow a display of the date in a particularly readable manner while contributing to the aesthetic appearance of the timepiece.
Therefore, the display device according to the invention, which is intended to be fitted to a timepiece of the type including a movement, a dial and hands, the latter being driven by the movement and displaying the time, includes first and second display members, driven in rotation by the movement, intended to display respectively the units and the tens of the date, and a drive and positioning mechanism controlled by the movement and arranged so as to cause one or the other or both of the display members to jump forward once a day. It is characterized in that the first member is mounted on the movement, pivoting about an axis which is perpendicular thereto and bears an index, and in that the mechanism and the first member are arranged so that, when the mechanism causes the display of the units of the date to pass from a value n to n+1, n being an integer number comprised between 1 and 9, the first member travels, in the clockwise direction, through an angle equal to 30xc2x0, and in that, when n is equal to 1, the index is located in a position corresponding to the position which an hour hand, pivoting about the axis of the first member, would occupy, when it is one o""clock.
In order to assure the passage from 9 to 0, the mechanism and the first member are arranged so that the latter performs a jump of 90xc2x0 when n is equal to 0, so that the index passes from the  less than  less than nine o""clock greater than  greater than  position to the  less than  less than midday greater than  greater than  position, via the shortest route.
In a particularly advantageous embodiment, the first member is formed of a disc, bearing the index, and placed underneath the dial. The latter includes nine apertures disposed in an arc of a circle substantially concentric with the disc, the first aperture occupying a position corresponding to  less than  less than 1 o""clock greater than  greater than  and the following ones making an angle of 30xc2x0 with respect to each other, with reference to the central point of the arc of the circle, so that the last aperture is located in the  less than  less than 9 o""clock greater than  greater than  position, so that altogether the index of the first member is visible through the apertures.
If, in order to display the units, it seems advantageous to use an index whose position corresponds to that of an hour hand, the situation is different for the indication of the tens. Indeed, perception of a number of objects comprised between 0 and 3 occurs without any problem and at first glance. This is why, advantageously, the second display member is also formed of a disc mounted so as to pivot on the movement and which bears three indices disposed in an arc of a circle, and the dial includes three apertures which are also disposed in an arc of a circle and of the same radius as the arc formed by the indices. Further, the tens display member and the drive and positioning mechanism are arranged so that zero, one, two or the three indices are visible through the three apertures, depending upon whether the figure of the tens is equal to 0, 1, 2 or 3.
An aesthetically pleasing display requires precise positioning of the discs. This is why it is advantageous for the mechanism to include two star wheels with twelve teeth, respectively secured to the first and second members and each cooperating with a jumper spring to position it.
In order to permit an optimum drive which only requires a correction for the months of less than 31 days, the mechanism and the display members are arranged so that, when the units figure of the date is equal to 0 or comprised between 2 and 8, only the units disc is driven, when the units figure is equal to 9, the units and tens discs are both driven, when the units figure is equal to 1, and the tens figure to 0, 1 or 2, only the units disc is driven, and when the units figure is equal to 1 and the tens figure to 3, only the tens disc is driven.
In order to assure optimum driving of the device, the mechanism further includes:
a  less than  less than 24 hour greater than  greater than  wheel, completing one revolution per day, driven by said movement, and carrying a drive finger,
a lever, mounted so as to pivot on the movement, actuated by said drive finger and cooperating with a return spring, provided with first, second, third and fourth drive means and positioning means, the first drive means cooperating with the star wheel with twelve teeth secured to the first member and causing it to move forward by one step each day,
a date wheel assembly including a date star-wheel, with 31 teeth, making one step per day via the action of second drive means, and a day cam, cooperating with the lever positioning means and defining three levels, the first, upper, level, corresponding to the position occupied by the date wheel assembly on the 31st of the month, the second, median, level corresponding to the positions occupied by the date wheel assembly when the units figure of the date is equal to zero, or comprised between 2 and 8, or when the units figure is equal to 1 and the tens figure to 0, 1 or 2, and the third, lower, level, corresponding to the position occupied by the date wheel assembly when the units figure is equal to 9,
a drive flirt for the star wheel with twelve teeth secured to the second member, provided with a star wheel with ten teeth and driven through one step per day by the third drive means of the lever, with the exception of the day when the positioning means abut against the upper level of the cam, and arranged so that the flirt is released when the units figure of the date is equal to 9,
a retrograde unit for the tens including a star wheel with 31 teeth, kinematically connected to the star wheel with 31 teeth of the date wheel assembly, released when the lever positioning means abut against the upper level of said cam,
a return spring of the second member, wound each time that the drive flirt moves the second member forward and let down when the retrograde unit is released, and
a drive lever, secured in rotation to the first member and cooperating with the fourth drive means, to cause the star wheel with twelve teeth secured to the first member to move forward three steps each time that the lever positioning means abut against the lower level of the cam.